This invention relates to a fret saw blade and an automatic fret sawing machine provided with the fret saw blade.
It is well known in the art to cut grooves along lines preliminarily described on workpieces, particularly wood plates or plastic plates, by reciprocating a fret saw blade in its longitudinal direction, and in such a machine, an operator supports and feeds the workpiece by hands so that the workpiece is cut by moving the fret saw blade along the line described on the workpiece.
Recently, due to shortage of skilled operators and application of numerically controlled machine tools to wood working fields, it has been proposed to automate the feeding of the workpiece, the rotation of the fret saw blade, and the clamping or removing of the ends of the blade by clamping members. However, in these machines, the fret saw blade itself has not been improved and a conventional one has been used.
FIGS. 1a and 1b show one example of conventional fret saw blades, and the fret saw blade 10 shown therein comprises a plurality of spaced apart saw-teeth 11 each having a width T which corresponds to the width of the shank 12 of the fret saw blade 10 and this width T determines the width of the groove of a workpiece to be cut. The thickness S.sub.2 of the shank 12 (where the thickness B of the blade 10=the height S.sub.1 of a saw-tooth 11+S.sub.2) is predetermined so that the fret saw blade will have a certain stiffness for resisting the reaction force created when a groove is cut and for sufficiently bearing the tension acting on the fret saw blade in its longitudinal direction and created when the workpiece is cut.
In the case where the conventional fret saw blade shown in FIGS. 1a and 1b is applied to an automatic fret sawing machine in which the feeding of the workpiece, the rotation of the fret saw blade, and the clamping of the ends thereof are automated, it was found that the following problems exist.
In a profile controlled or numerically controlled automatic fret sawing machine, clamping members of the fret saw blade are angularly controlled so that the fret saw blade will be moved linearly or tangentially in accordance with the shape of a straight or curved line depicted on the workpiece to be cut which is predetermined by programmed profile information. However, it was found that the acutally cut groove somewhat deviates from the desired line in the course of the cutting operation in spite of the angular control of the clamping members, as shown in FIG. 2 wherein line AB is a desired line and line AB' is an actually cut line. This is because the direction of advance of the fret saw blade is affected by the fiber orientation of the workpiece such as a wood plate or bamboo plate. This fact will be explained hereunder in conjunction with FIGS. 3 and 4.
In FIG. 4, clamping members 13 and 14 rigidly clamp the upper and lower ends of the fret saw blade 10, and although the clamping members 13 and 14 can be controlled to direct the fret saw blade to the correct feeding direction by means of a profile control or numerical control system, the blade 10 is gradually directed to the fiber orientation Z of the workpiece M during the cutting thereof (shown in FIG. 3). When the workpiece M is fed in the direction of X as shown in FIG. 3, although the fret saw blade 10 is at first disposed in parallel to the direction X, it is then subjected to a feeding force F which creates a cutting resistance, and a component force Fa of this cutting resistance F acts upon the lower side (as viewed in FIG. 3) of the fret saw blade 10 because of the fiber orientation of the workpiece, the orientation being illustrated as oblique lines in FIG. 3. Thus, the fret saw blade 10 is inclined by the component force Fa and is deflected by .delta. in the direction normal to the direction of feeding the workpiece as shown in FIG. 4. The deflection .delta. of the fret saw blade 10 causes that the profile which is formed after cutting the workpiece will not coincide with the desired profile based on the programmed instructions. However, this deflection .delta. does not exceed a certain extent because of a restoring force generated by the increase in the twisting stress and the bending stress of the blade 10 caused by the increase in the deflection .delta. and because of a set effect of a saw-tooth of the blade 10 at the corner portions Q thereof when the blade is inclined with respect to the direction of feeding of the workpiece M. In our experiment, the deflection .delta. reached 2-3 mm with respect to a fret saw blade having a width of 0.7 mm. If the fret saw blade were used under the deflected condition, the blade would be broken by the repeated bending stress and the frictional heat generated at the contacting surface between the side surfaces of the fret saw blade 10 and the workpiece M to be cut, and even when a guide 15 is used as shown in FIG. 5, the blade would be broken by the repeated bending stress and the frictional heat at the contacting surface between the guiding surface of the guide 15 and the blade 10. Furthermore, where a cut groove K is not formed vertically to the surface of the workpiece as shown in FIG. 6 when the fret saw blade is used under the bent condition, and if such a cut groove K as shown in FIG. 6 is used as a groove for a cutting blade for use in die making, it is impossible to produce a product with high quality.
In our further experiment, a nine-ply plywood having a thickness of 18 mm which is used for the die making was used and a fret saw blade shown in FIG. 1 was also used to observe how the fret saw blade would be deflected by the fiber orientation of the workpiece to be cut.
FIG. 7a shows a pattern according to which above described plywood is cut radially at a spacing of 15.degree. in the X-Y plane in which h.sub.1 through h.sub.24 and H.sub.1 through H.sub.24 are holes for inserting the fret saw blade 10 to start the cutting of the grooves, and dotted lines interconnecting inner and outer holes represent lines to be cut by programmed instructions. The fret saw blade starts to cut the groove from an inner hole along the dotted line to an intermediate point P.sub.1, for example, and then from an outer hole to the intermediate point P.sub.1. However, for example, as to the line connecting h.sub.1 to H.sub.1, an actually cut groove does not coincide with the predetermined line connecting h.sub.1 to H.sub.1 through P.sub.1 as shown in FIG. 7b for the reason described hereinafter.
FIG. 8a shows one example of a plywood having nine plies which are laminated such that the fiber orientations of odd-numbered plies are substantially perpendicular to the fiber orientations of even-numbered plies, and it is presumed that the fiber orientations substantially correspond to Y and X axes of rectangular coordinates respectively as shown in FIGS. 8b and 8c.
In the case where such a plywood is cut as shown in FIG. 7a, although the following is not absolutely correct because the fiber orientations are not always disposed uniformly, it was found that the deviation of an actually cut line from the predetermined line to be cut reaches a maximum value at angles of 15.degree., 75.degree., 105.degree., 165.degree., 195.degree., 255.degree., 285.degree. and 345.degree., reaches an intermediate value at 30.degree., 60.degree., 120.degree., 150.degree., 210.degree., 240.degree., 300.degree. and 330.degree., and reaches a minimum value at 0.degree., 45.degree., 90.degree., 135.degree., 180.degree., 225.degree., 270.degree. and 315.degree. in the rectangular coordinates in which the line h.sub.12 -H.sub.12 (h.sub.24 -H.sub.24) corresponds to the X-axis and the line h.sub.6 -H.sub.6 (h.sub.18 -H.sub.18) corresponds to the Y-axis. The reason why such various deflections of the cut lines are caused will be described hereunder.
FIG. 9 shows an ideal lattice-like modeled pattern of the fiber orientations of the plywood. It will be understood that when the cutting operation starts from the center of the plywood in a radial direction, the fret saw blade 10 is affected and deflected by the fiber orientation in the direction of X in the shaded area in FIG. 9 and by the fiber orientation in the direction of Y in the area not shaded respectively. Accordingly, the cutting lines h.sub.1 -P.sub.1 ' and H.sub.1 -P.sub.1 " do not coincide with lines h.sub.1 -P.sub.1 and H.sub.1 -P.sub.1 as shown in FIG. 7b.
Usually when a pattern depicted on the surface of a workpiece such as a wood plate is cut by using a jig saw, the cutting operation has been performed while observing and correcting the deviation between the actually cut line and the preliminarily designated profile by manually controlling the feeding of the workpiece. However, some of automatical fret sawing machines which are operated by a profile control or numerical control device have not a function of controlling the deviation and although an automatic fret sawing machine provided with a controlling mechanism correcting the deviation may be technically possible, such machine is expensive and not economical.
Furthermore, recently, there has been proposed a fret saw blade 10 having saw-teeth 11 with alternate sets of teeth offset transversely as shown in FIGS. 10a and 10b.
Although such fret saw blade 10 is advantageous to cut a metal workpiece, it is useless for cutting a workpiece having elasticity such as a wood plate or bamboo plate for the reason that in the cutting operation of these fibrous materials, when the cutting quality is lowered, the elastic deformation degree of the portion of the workpiece engaging the fret saw blade and the elastic restitution degree at the cut groove of the workpiece after the cutting are larger than those in the case where the fret saw blade has a good cutting quality. For this reason, the width of the cut groove is reduced, and the reduction and the nonuniformity of the width of the cut groove make it improper to put a cutting blade for use in die making in the groove. Moreover, it is difficult to make the edge line 10a-10a of a saw-tooth to be precisely perpendicular to the axis 10.sub.c -10c of the fret saw blade 10 and to accurately set the dimension of the sets of the saw-teeth so as to be bilaterally symmetrical (i.e. T.sub.1 =T.sub.2) as shown in FIG. 11, and the whole fret saw blade tends itself to be offset with respect to its longitudinal axis. Therefore, as far as an automatic fret sawing machine which uses a fret saw blade provided with such sets of the saw-teeth as shown in FIGS. 10a, 10b is concerned, it is almost difficult to cut a groove coinciding with a predetermined line to be cut, and more particularly, circles cut clockwisely and anti-clockwisely in accordance with the same programmed instructions would have different diameters. Thus, it is impossible to use a conventional fret saw blade provided with sets of the saw-teeth for an automatically controlled fret sawing machine. Furthermore, it is required that the conventional fret saw blade is provided with an even-number of sets engaging the workpiece in cutting operation to balance the cutting line.